This invention relates to ignition coil devices for internal combustion engines by which an ignition coil and a control circuit are accommodated in the accommodating chamber and the pocket of a mold resin casing, respectively.
FIG. 1 is a circuit diagram showing the fundamental circuit structure of a prior art ignition coil device for an internal combustion engine. The circuit includes an ignition coil 1 and an igniter 2, which consists of a control unit 2a and a power transistor 2b. FIG. 2 is an end view of a prior art ignition coil device disclosed in Japanese Patent Application No. 63-255831; FIG. 3 shows a section of an ignition coil device of FIG. 2 along the line III--III of FIG. 4; and FIG. 4 is a side view of an ignition coil device of FIG. 2. A heat sink 4 is attached at its bottom fixing plate 41 to the core 3. The core 3 and the heat sink 4 are secured to a mold resin casing 5, which encloses an accommodating chamber 51 therein. The mold resin casing 5 exposes via its windows 53a and 53b portions 42a and 42b of the heat sink 4. A power transistor unit 6 (implementing the igniter 2 of FIG. 1) is accommodated within a pocket 52 of the mold resin casing 5.
The coil assembly 7 includes a primary winding wound on the primary winding bobbin 71a and a secondary winding wound on the secondary winding bobbin 71b. A high voltage tower 8 is provided with terminals for leading out to the exterior the high voltage induced in the secondary winding of the coil assembly 7. A connector 9 provides an electrical connection from the power transistor unit 6 to exterior circuits. By the way, a resin material is filled in the remaining space within the accommodating chamber 51 and the pocket 52 of the mold resin casing 5.
The method of operation of the above ignition coil device is as follows. In response to control signals supplied via the connector 9, the power transistor unit 6 turns off the primary current flowing through the primary winding of the coil assembly 7. The high voltage induced in the secondary winding is supplied via the high voltage tower 8 to a distributor of the internal combustion engine.
The heat generated in the power transistor unit 6 is transmitted through the heat sink 4 and is radiated from the exposed portions 42a and 42b via the windows 53a and 53b, or directly from the core 3. The heat generated in the primary winding and the secondary winding is primarily radiated from the core 3.
The above ignition coil device has the following disadvantage.
Since the heating generating power transistor unit 6 and the coil assembly 7 are disposed close to each other within a compact casing, the heat generated by them is accumulated. The power transistor unit 6 and the coils of the coil assembly 7 are heated to a high temperature and may thus burn and fail.
FIG. 5 is a view similar to that of FIG. 3, but showing another prior art ignition coil device; and FIG. 6 shows a section along the line VI--VI of FIG. 5. The structure of the ignition coil device of FIGS. 5 and 6 is similar to that shown in FIGS. 1 through 4, where like reference numerals represent like parts. However, the primary winding 7a and the secondary winding 7b wound on the primary winding bobbin 71a and the secondary winding bobbin 71b, respectively, are shown explicitly in FIG. 6.
The ignition coil device of FIGS. 5 and 6 has the following disadvantage.
A too large inductance of the core 3 slows down the rising speed of the secondary current. Thus, for the purpose of decreasing the inductance of the core 3 and thereby increasing the rising speed of the secondary current, a spacer 3a is inserted across a leg of the core 3. This insertion of the spacer 3a entails increase in the production cost and the number of assembly steps.
FIGS. 7 through 9 show still another prior art ignition coil device similar to that shown in FIGS. 1 through 4, like reference numerals representing like parts.
FIG. 10 is an exploded view of another, conventional ignition coil device similar to that of FIGS. 7 through 9. The high voltage tower 8 is attached at the high voltage tower attachment portion 8a to the secondary winding 7b of the secondary winding bobbin 71b. The connector 9 is attached at the connector attachment portion 9a to the bobbin attachment portion 7c of the primary winding bobbin 71a. The assembling of the ignition coil device of FIG. 10 is effected as follows.
First, the high voltage tower attachment portion 8a of the high voltage tower 8 is engaged with the bobbin attachment portion 7d on the secondary winding bobbin 71b, such that the high voltage tower 8 and the secondary winding 7b wound on the secondary winding bobbin 71b form an integral unit. The high voltage tower 8 and the secondary winding 7b are electrically connected to each other via solder. Second, the connector attachment portion 9a of the connector 9 is engaged with the bobbin attachment portion 7c on the primary winding bobbin 71a, such that the connector 9 and the primary winding 7a wound on the primary winding bobbin 71a form an integral unit. The connector 9 and the primary winding 7a are electrically connected to each other via solder. The first and the second integral units thus obtained are accommodated within the mold resin casing 5, and, thereafter, electrical connections between the connector 9 and the secondary winding 7b and between the high voltage tower 8 and the secondary winding 7b are effected.
The ignition coil device of FIG. 10 has the following disadvantage.
The electrical connections between the connector 9 and the secondary winding 7b and between the high voltage tower 8 and the secondary winding 7b must be effected after the first and the second integral units are accommodated within the mold resin casing 5. Much time is needed for the interior wiring operations, and hence the assembling efficiency is low.
FIG. 11 is a view similar to that of FIG. 4, but showing still another conventional ignition coil device; and FIG. 12 is a schematic sectional end view of the ignition coil device of FIG. 11. The ignition coil device of FIGS. 11 and 12 is also similar to that of FIGS. 1 through 4, where like reference numerals represent like parts. However, FIG. 12 shows the electrical connections to the power transistor unit 6 explicitly. A first terminal 10 of the power transistor unit 6 is coupled to a drive signal line 11 forming an interior wiring for controlling the primary winding 7a. A second terminal 12 of the power transistor unit 6 is connected to the ground line 13 forming part of the interior wiring of the ignition coil device. A third terminal 14 of the power transistor unit 6 is coupled to the primary winding 7a.
The ignition coil device of FIGS. 11 and 12 has the following disadvantage.
Since the ground line 13 runs through the interior of the core 3 forming the magnetic path of the ignition coil device, a voltage is induced by the flux passing through the core 3. If, for example, a negative voltage is induced in the ground line 13 at the time when an output voltage is generated in the secondary winding 7b, the power transistor unit 6 may be turned on due to the lowering voltage at the ground line 13. The output voltage of the secondary winding 7b may thus be reduced.